Defects in solids

All solids, even the most ‘perfect’ crystals contain defects. Defects are of great importance as they can affect properties such as mechanical strength, electrical conductivity, chemical reactivity and corrosion. There are several terms used to describe defects which we must consider:

Intrinsic defects – present for thermodynamic reasons.

Extrinsic defects – not required by thermodynamics and can be controlled by purification or synthetic conditions.

Point defects – Occur at single sites. Random errors in a periodic lattice eg absence of atom from usual place (vacancy) or atom in a site not normally occupied (interstital).

Extended defects – ordered in one, two and three dimensions. Eg errors in the stacking of planes.

Every solid has a thermodynamic tendency to acquire point defects, as they introduce disorder and therefore increase entropy.

The Gibbs free energy, G = H – TS, of a solid, is contributed to by the entropy and enthalpy of the sample (fig. 14). Entropy is a measure of disorder within a system, hence, a solid with defects has a higher entropy than a perfect crystal.